Digital video cameras are devices that produce digital image data representative of a period of time in a scene. As used herein ‘digital video camera’ refers to any camera that generates such image data. ‘Video’ as used herein refers generally to a series of frames and associated timing information. The term video is used to refer to both a video display, i.e. the display of streamed frames, and also to video data, i.e. the digital information which may be stored or used to produce a video display. The term ‘frame’ as used herein refers to a single complete still image in a sequence of images that creates the illusion of motion within a scene when displayed in rapid succession (streamed). ‘Frame’ is also used to refer to digital information representative of the single still image. Frames within video are associated with a brief period of time equal to 1/fps. The term ‘fps’ is an abbreviation for frames per second; as used herein, ‘fps’ refers to the rate of video playback and/or recording speed of video.
A frame rate of 30 fps is often referred to in the art as ‘real-time speed of video’ because it has a high enough fps that the video appears ‘smooth’ to the human eye. For the Phase Alternating Line (PAL) standard established by the Nation Television Standards Committee adopted in Europe, the ‘full-motion rate’ is defined as 25 fps, each frame containing 576 horizontal lines by 768 pixels (i.e., there are 25 frames in each second of video, resulting in each frame representing 1/25 seconds of video). For the National Television Standard Committee (NTSC) standard adopted in America and Japan, the ‘full-motion rate’ is defined as 29.97 fps, each frame containing 480 horizontal lines by 640 pixels. A rate of 24 fps is generally considered acceptable for animation. At a slower rate of 12-15 fps, video appears ‘choppy’ to the human eye if moving objects are in the scene.
The video generated by a digital video camera may be transmitted to a memory device for storage or to an output device such as a video monitor or television that produces a replicated video of the sequential images of the scene.
Digital video cameras typically have optical elements, at least one two-dimensional photodetector array, a data storage device, a controller and a display, all of which are generally mounted to a camera housing. Non-limiting examples of digital video cameras are described in the following patents which are hereby incorporated by reference for all that is disclosed therein: U.S. Pat. No. 6,097,879 of Komatsu et al. issued on Aug. 1, 2000 under the title VIDEO CAMERA APPARATUS OF DIGITAL RECORDING TYPE and U.S. Pat. No. 5,343,243 of Maeda issued on Aug. 30, 1994 under the title DIGITAL VIDEO CAMERA.
The digital video camera optical elements serve to focus an image of a scene onto the camera's two-dimensional photodetector array. The optical elements typically comprise one or more lenses and/or reflectors. The two-dimensional photodetector array generates image data representative of the image of a scene imaged thereon. The controller serves to process the image data and to transfer the image data to and from the data storage device. The controller also serves to transfer image data to the display and/or an output device. Additionally, the controller may serve to sequence this image data for playback at a later time to produce video.
The two-dimensional photodetector array has a plurality of photodetectors typically arranged in closely positioned rows and columns. Each photodetector generates image data representative of a small portion of the optical image of the object focused on the photosensor array. The accumulation of image data generated by the plurality of photodetectors is representative of the entire image of the scene, similar to a mosaic representation of the image of the scene. Each photodetector outputs a data value which corresponds to the intensity of light it receives. The controller processes and arranges the image data generated by the plurality of photodetectors into a complete set of image data often referred to as a frame of a video. As sequentially captured sets of image data are collected, stored and/or played, they produce video.
Digital video cameras process frames at a relatively high rate of speed. As previously discussed, the frames are recorded and/or played at a frame rate (e.g., 30 fps). Each frame and timing data associated therewith, when stored separately, may require a relatively small amount of memory. However, when frames are stored sequentially as video, the sequence of frames often requires a large amount of data. Storage of this video may consume memory reserves relatively quickly when the frame rate is relatively high (e.g. 30 fps). Additionally, capturing, processing, storing and playing image data requires a significant amount of power. Portable digital video cameras have power supplies that have limited power storage (e.g. batteries), therefore only a limited amount video capture may be completed before stored power is depleted.
Conventional video cameras capture frames of video at a predetermined frame rate. This predetermined frame rate is preset at the time of manufacture or set by the user as a one-time-event. This one-time setting of the frame rate occurs prior to capturing video. Such one-time-event setting of frame rate may, for example, be at a lower fps setting (e.g. 6 fps), a higher fps setting (e.g. 40 fps), or the usual setting of about 30 fps. The lower fps setting is typically used in order to conserve memory and power reserves. The relatively high fps setting (e.g. 40 fps) is typically used in order to better capture scenes in which action occurs.